Austenite type heat-resisting steel having high strength at an elevated temperature and the process for producing same

ABSTRACT

AN AUSTENITE TYPE HEAT-RESISTANT STEEL, CONTAINING IN WEIGHT PERCENT FROM 0.1 TO 1% CARBON, FROM 0.01 TO 3% SILICON FROM 0.01 TO 10% MANGANESE FROM 13 TO 35% CHROMIUM, FROM 15 TO 50% NICKEL, AND THE BALANCE ESSENTIALLY IMPURITES AND IRON, OR ADDITIONALLY OTHER ELEMENTS SUCH AS CO, W, MO, NB, TI, AL AND/OR N, WHICH IS CHARACTERIZED BY HIGH STRENGTH AT ELEVATED TEMPERATURES, AND WHICH IS ADAPTABLE TO FORMING BY FORGING OR CENTRIFUGAL CASTING PROCESSES, IS PROVIDED HEREIN.

July 30, 1974 SADAQ OHTA E AL 3,826,689

AUSTENITE TYPE HEAT-RESISTING STEEL HAVING HIGH STRENGTH AT AN ELEVATED TEMPERATURE AND THE PROCESS FOR PRODUCING SAME Filed. March 9. 1972 2 Sheets-Sheet 1 X Cr: 25%

|280C lOmin. (I hr.) 900C-W.Q. x I o o X 1 STRUCTURE I A I in II Q3 x A o o 0 I 111 5 o Z LlJ E o 05 x A u o o O O.2- X o o o O X X ,3 3 X X X X l l 0 IO 40 Ni CONTENT ("/o) (Kglmm 2.2 og U 2.0 L Tr" E g "0- STRENGTH OF CAST a 8 L8 STEEL N0.1OO CL 0 B5 |.e I E g 1.4 o a; 1.2-

| 1 I 1 O 20 4o I00 GRAIN BOUNDARY OCCUPYING RATlO ("/o) y 1974 SADAO OHTA E L 3,826,689

AUSTENITE TYPE HEATQULSISTIH G HAVING HIGH STRENGTH AT A" ELEVATED TEMPERATURE AND THE PROCESS FOR PRODUCING SAME Film-larch 9. 1972 2 Sheets-Sheet n FIG.2(]I) United States Patent US. Cl. 148-3 5 Claims ABSTRACT OF THE DISCLOSURE An austenite type heat-resistant steel, containing in Weight percent from 0.1 to 1% carbon, from 0.01 to 3% silicon, from 0.01 to manganese, from 13 to 35% chromium, from 15 to 50% nickel, and the balance essentially impurities and iron, or additionally other elements such as Co, W, Mo, Nb, Ti, Al and/or N, which is characterized by high strength at elevated temperatures, and which is adaptable to forming by forging or centrifugal casting processes, is provided herein.

BACKGROUND OF THE INVENTION Field Of The Invention This invention relates to an austenite type heat-resistant steel which is capable of use at elevated temperatures of over about 750 C. This invention further relates to a process for producing the same.

Description Of Prior Art Recent developments in the petrochemical industry, particularly in the production of ammonia, methanol, ethylene or the like, has resulted in a demand for steels which are capable of withstanding high temperatures, in the order of over 750 C., for extended periods of time.

Heretofore, 0.0SC-Cr20 Ni stainless steel and 20Cr 32Ni-0.5Al-0.5Ti-Fe alloy (Incolloy 800 alloy) have found wide use for applications involving temperatures of below 850 C. These alloys, however, have suffered from a variety of shortcomings. For one, since these alloys contain a relatively low carbon content, their strength is considerably reduced and they are generally incapable of withstanding a sustained load at such elevated temperatures. Although these shortcomings can be avoided to some extent when high carbon-high chromium-high nickel stainless steels are used as castings produced by the cen trifugal casting process, the use of that technique does not alleviate the entire problem. Tubes produced by the centrifugal casting process are characterized by high strength at elevated temperatures, but they cannot be formed into small diameter shapes of less than 75 mm. OD. and wall thicknesses of below 6 mm. Moreover, they cannot be formed into tubes of lengths of over 2 m. with diameters of 75 mm. O.D. nor tubes of lengths of over 4 m. with diameters of over 120 mm. O.D. Furthermore, tubes formed by that method have generally inferior inner surfaces and, of course, that technique cannot be used to produce sheets.

Previous attempts have been made to improve the high temperature strength of high carbon-high chromium-high nickel stainless steel, by use of a solution heat treatment in which, after the steel has been hot or cold worked, it is heated to a temperature of from 1,000 to 1,300 O, and then water quenched. These efforts, however, have met with only limited success in terms of creep rupture strength, and the strength is only slightly improved as compared with steels of low carbon contents, such as ice 0.50C-25Cr20Ni stainless steel or 20Cr32Ni0.51Al-0.5 Ti-Fe alloy.

Other attempts at improving high temperature strength properties include the addition of Co, W, Nb, Ti, Al or the like, to the forging alloys, for instance, to such steels as high Cr-high Ni stainless steel having less than 0.1% level of carbon. This has been shown to be satisfactory in certain instances to provide strength characteristics comparable to that of high C-high Cr-high Ni stainless cast steels. Nevertheless, those alloys are generally characterized by poor hot or cold workabality, so that they are diflicult to form into long length, small diameter tubes.

A need continues to exist, therefore, for a high temperature, heat resistant steel having high strength properties at the higher temperature ranges which possesses good workability.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a high Cr-high Ni heat resistant steel having high strength properties at temperatures of above 750 C.

It is another object of the invention to provide high Crhigh Ni heat resistant steel which is suitable for use in forgings, rollings, or extruded forms with the superb characteristics which permit the production of tubes having improved wall thickness and length and the production of sheets.

It is a still further object of the present invention to provide a process for producing such a high Cr-high Ni heat resistant steel.

These and other objects, as will hereinafter become more readily apparent, have now been attained by the discovery that the high Cr-high Ni heat resistant steel having high strength characteristics at elevated temperatures and which has desirable forging, rolling or extrusion characteristics, can be obtained by the use of a novel alloy. In particular, it has now been discovered that the creep rupture mechanism of a steel used at elevated temperatures ranging from 800 C. to 1,000 C. appears to differ from that of a steel used within a temperature range of from 600 to 700 C. At temperatures above 750 C., say, at 1,000 C., in order to attain improved creep rupture strength, it is necessary for the carbide to precipitate on the grain boundaries in a continuous or partially-continuous form. The ratio of the carbides occupying the grain boundaries (the grain boundary occupying ratio) should nevertheless be over 30%, preferably over 50%. These facts have led to the discovery of an alloy steel wherein the carbides appearing on the grain boundaries are precipitated in continuous or partially-continuous form. This steel alloy contains carbon, silicon, manganese, chromium, nickel, and the balance essentially impurities and iron.

This steel alloy is produced by a novel process which includes a unique heat treatment technique wherein the steel is heated to a temperature in the range of from 1,l50 C. to the solidus temperature, and it is then cooled to a temperature of 950 C. or higher. Subsequent to this solution heat treatment, after the casting or working of the steel, the slow cooling is followed by water quenching.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical representation of austenite type stainless steel compositions containing from 0.004 to 1.2% carbon, about 25% chromium and from 8 to 50% nickel;

FIG. 2 are photomicrographs of selected composition structures wherein FIGS. I and II are provided for purposes of comparison with structure III of the present invention. FIG. IV shows a structure having a carbide precipitate dispersed within the grains; and,

FIG. 3 is a graphical representation of the ratio of grain boundary occupation of carbide and its effect on creep strength.

3 DETAILED DESCRIPTION OF THE INVENTION In greater detail, the present invention provides an austenite type heat resistant steel having high strength properties at elevated temperatures, and containing, in weight percent, from 0.1 to 1% carbon, from 0.01 to 3% silicon, from 0.01 to manganes, from 13 to 35% chromium, from 15 to 50% nickel, and the balance essentially impurities and iron. This alloy is characterized by a precipitation of carbides on the grain boundaries in a continuous or partially-continuous form.

The present invention further provides an austenite type heat resistant steel containing, in weight percent, in addition to the alloy elements referred to above, at least one element selected from a group of less than 30% cobalt, less than 10% tungsten, less than 10% molybdenum, less than 5% niobium, less than 5% titanium, less than 5% aluminum and less than 0.5% nitrogen.

Furthermore, the present invention presents a process for producing an austenite heat resistant steel having the above-defined composition which is subjected, subsequent to casting or working, to a heat treatment at a temperature of from 1,150 C. up to the solidus temperature, followed by slow cooling from said temperature to a temperature of 950 C. or higher, for a time of from 5 seconds to one hour, and thereafter water quenching.

Referring now to FIG. 2, photomicrographs of austenite type stainless steel structures having a variety of compositions containing from 0.004 to 1.2% carbon, about chromium and from 8 to 50% nickel are shown. Those compositions were subjected to heat at a temperature of 1,280 C. for 10 minutes, slow cooling from 1,280 C. down to 950 C. for one hour, and then water quenching to room temperature.

Table I further describes the structures shown in FIG. 2 and indicates the creep rupture strength of the various austenite type compositions.

FIG. 2 (I) shows a structure having no carbide precipitate on the grain boundaries; (II) shows carbide precipitate aligned partially on the grain boundaries, with a grain boundary occupying ratio of up to (III) shows a structure having a continuous carbide precipitate on the grain boundaries, and (IV) shows a structure having a carbide precipitate dispersed within the grains. As shown in Table I, sample No. 101 of the structure (I) where no carbide precipitation occurred on the grain boundaries, comparatively inferior creep rupture strength was obtained. In Sample No. 102 of structure (II), carbide was precipitated only in partial alignment with the grain boundaries and likewise showed a comparatively low creep rupture strength. In contrast, Samples Nos. 102 to 109 of the structure (III), wherein carbide precipitation occurs on the grain boundaries, improved creep rupture strength was obtained.

FIG. 3 shows the result of a series of tests made to clarify the efiFect of the quantity ratio of carbide precipitation on the grain boundary on the creep rupture strength of steels containing 0.4% carbon, 25% chromium, and 20% nickel.

This figure clearly indicates that the higher the quantity ratio of carbide precipitation in grain boundary, the higher will be the creep rupture strength. If the quantity ratio falls to less than 20%, however, the creep rupture strength will be only moderately increased. If the quantity ratio in the grain boundary is more than 30%, the creep rupture strength will be greatly improved and will be substantially equal to that of cast materials. Moreover, the creep rupture strength will be superior to that of cast materials if quantity ratio in the grain boundary exceeds From the above results, it is concluded that the ratio of grain boundary occupied by carbides should be over 30%, and preferably over 50%.

With particular reference to the alloying elements such as C and Ni which play an important role from the structural viewpoint, it can be appreciated from FIG. 1 that if the carbon level is less than 0.1%, such as in structures (I) and (II), the creep rupture strength will be extremely inferior, whereas if the carbon level is over 0.1% as shown in (III) of Table I, the creep rupture strength will be greater. However, if the carbon level is over a certain limit, the creep rupture strength will be reduced conversely, as shown in Table I. Furthermore, the higher the carbon content, the lower the cold workability. With these factors in mind, the upper limit of the carbon content has been set as 1%.

The amount of Ni can vary from between 15 and 50%. Table 2 shows the creep rupture strength at 1,000 C., of a stainless steel containing about 0.4% carbon, about 45% chromium, and from 8 to 45% nickel. This steel had been subjected to heat treatment at 1,280 C. for 10 minutes, and was then slowly cooled from 1,280 C. down to 950 C. for one hour, followed by rapid cooling to room temperature. Samples 201 and 202 represent structure (I) characterized by a low creep rupture strength, while samples 203 to 208 containing over 15.7% Ni, represent structure (III) which is characterized by greatly improved creep rupture strength. The higher the nickel content, the higher will be the creep rupture strength. However, if the nickel content exceeds 50%, no further improvement in strength will be obtained, and it will only be adding to the costs.

TABLE 1 1,000 hr. creep rupture Alloy element, percent strength Sample (kg/mm?) number 0 Cr Ni Stmcture at 1,000 C.

0.40 24.91 19.92 I 1.90 0.04 25.18 20.40 II 0.90 0.08 25. 32 19.99 III 0.95 0.14 24.69 20.38 111 1.65 0. 20 25.04 19.57 III 1.70 0.31 25.46 20.59 III 1.90 0.43 25.82 20.41 III 1.95 0.65 25.18 20.23 III 1. 0.82 25.45 19.74 III 1.90 0.95 24.71 20.16 III 1.80 1.15 25.38 20.21 III 1.65

TABLE 2 1,000 hr. creep rupture Alloy element, percent strength Sample (kg/mini) number 0 Cr Ni Structure at 1,000 C.

0.39 25.24 8.29 I 0.85 0.42 25.56 13.51 I 0.90 0.47 25.38 15.70 III 1.85 0.43 25.80 20.46 111 1.95 0.35 25.42 24.82 III 2.05 0.39 24.75 30.67 III 2.00 0.42 24.96 35.53 III 2.10 0.37 25.18 45.40 III 2 00 Silicon may be used within the range of 0.01 to 3%. Silicon is added to the molten steel during production for the purpose of deoxidation. Although silicon improves the oxidation resistance of the steel for use at elevated temperatures, greater than 3% will impair weldability and workability, and enhance the formation of a sigma phase.

Manganese may be present in amounts of from 0.1 to 10%, also, for the purpose of deoxidation during steel making. Manganese tends to stabilize the austenite and to provent the formation of a sigma phase. If more than 10%, however, is used, oxidation resistance is reduced.

Chromium may be present in amounts of from 15 to 35%, also to improve the oxidation resistance of the steel. For product use at temperatures of over 750 C., the chromium content should be at least 15%. However, if greater than 35% is used, such larger amounts can cause difficulties in hot or cold working, and also increase the tendency of sigma phase formation.

To further improve the creep rupture strength at elevated temperatures, Co, W, Mo, Nb, Ti, Al and/or N, may be further added.

Cobalt should be present in amounts of less than 30%. Cobalt will completely dissolve in the matrix of high Cr-high Ni austenite to greatly improve the creep rupture strength. If the level exceeds the cost of the steel will be increased unfavorably.

If tungsten is used, the quantity present is limited to amounts of below 10%. Tungsten, together with molybdenum, dissolve at the solution heat treatment temperature in the austenite matrix, with a part thereof precipitating in the form of carbides during the slow cooling. The greater amount, however, will remain in solid solution, so that during use, it will precipitate as fine carbide particles within the grains to thereby increase the product strength. However, even if the level is increased beyond 10%, there will be no substantial increased effectiveness, and the hot or cold workability can be impaired.

in FIG. 2 (IV). As can be observed, no precipitate occurred on the grain boundaries. It is advantageous, in a sense, that the higher the temperature of the solution heat treatment, the shorter will be the time required for the solution of the carbides. However, if the temperature reaches the solidus line of the steel, the steel is likely to melt. Accordingly, the temperature of the solution heat treatment is limited to the range of from 1,150" C. to the solidus line.

Table 4 compares the structures of steels according to the present invention with certain controls. Those samples were heated at 1,280 C. for 10 minutes after forging, and then slowly cooled to 950 C. for a time ranging from 1.1 second to 1.5 hour.

TABLE 4 Alloy element, percent Cooling time from 1,280 0. down to 950 C. (X)

Sample 1.1 2.4 4.1 5.7 15.5 28.3 1.1 4.3 2.85 1 1.5 number 0 Cr Ni see. sec. sec. see. sec. see. min. min. min. hr. hr.

0.04 18.27 8.36 I I I I I I I I I I I. 0.58 17.98 8.01 I I I I I I I I I I II. 0.07 18.16 12.11 I I I I I I I I I I I. 0.78 18.03 12.28 I I I I I I I I I II II. 0.05 20.33 15.27 I I I I I I I I I I I. 0.62 19.85 15.53 I I I I II II II III III III III. 0.08 25.32 19.99 I I I I I I I I I I I. 0.14 24.59 20.38 I I I I I I I II II III III. 0.20 25.04 19.57 I I I I II II III III III III III. 0.43 25.82 20.41 I I I II III III III III III III III. 0.95 24.71 20.16 I I II III III III III III III III III. 1.15 25.38 20.21 I I II III III III III III III III III. 0.04 20.21 31.82 I I I I I I I I I I I. 0.13 20.16 32.05 I I I II II III III III III III III. 0.28 19.94 32.33 I I I II III III III III III III III. 0.57 20.01 32.17 I I II III III III III III III III III. 0.04 25.07 47.80 I I I I I I I I I I I. 0.17 24.85 47.52 I I II II III III III III III III III. 0.28 24. 91 48.33 I II II III III III III III III III III.

Niobium may be present in amounts of below 5%. It has been a common practice in producing forgings, The addition of niobium is intended to permit the carsuch as producing in forgings of 25 Cr-20Ni type stainbides to precipitate within the grain boundaries during less steel having a carbon level of below 0.1% or in prothe use of steels to thereby improve strength. However, if ducing forgings of Incolloy 800 alloy, or the like, to use the quantity of niobium used is over 5%, the hot workaa solution heat treatment in which the forgings are water bility and weldability will be greatly lmpaired. quenched immediately after the heating at a solution Titanium may be present in amounts of below 5%. Titreatment temperature. However, as shown in Table 4, the tanium tends to precipitate, durmg the use at an elevated structure (I) is obtained when subjected to conventional p r as carbides r N 3 wlthm h g m solution heat treatment in which the forgings are imboundaries. However, with a level of over 5%, hot workamediately water quenched from a temperature of 1,280 bility of the product will be greatly impalred- C. to 950 C. within a period of 1.1 seconds. In such a Alumlnum may e pr e 1n 3111011018 9 below 5 conventional solution heat treatment, the structure (III) Alumlmlm tends to Precipltate 3 dul'lng use at cannot be achieved, even if the carbon level is increased Vated temperatures 1 thereby Increase strength 9 to greater than 1%. Alternatively, when the steel is tha Stefil- However, 1f the qq of PP 15 cooled by means of helium gas or cool air injection from over the hot werkablhty and weldeblhty W111 be 1,280 (3. down to 950 c. in a period of 2 to 4 seconds, p f only the structure up to (II) can be obtained even with g m y p i 111 amounis of p to the steel containing high levels of C and Ni. In contrast, Nitrogen tends to dissolve 1n the matrix of the austemte when cooling is carried out over Period f more than 5 to increase Strength of the Steel- Howeveft 1f grFater seconds, the structure (III) is obtained at a carbon conthan N F g Pores develop the mgot tent of 0.9% or higher in case of 25CR-20Ni steel, at a thus causmg dlmcumes m Producmg good Steel carbon content of 0.5% or higher in case of 20Cr-32Ni HEAT TREATMENT ACCORDING TO THIS steel and at a carbon content of 0.2% or higher in case INVENTION of 25 Cr47Ni steel, respectively. Accordingly, cooling Table 3 describes the structures of steels according to Should occur over a penod of mme than 5 secojlds the present invention, which have been heated to a tom Table 5 shows creep rupture Stmngths at 1,000 for perature of from 1,050 to 1,350" C. for 30 minutes, subsequent to the forging, and then have been slowly cooled to 950 C. for one hour and then water quenched.

samples as shown in Table 4. The results of the tests are also provided to show the desirability of slower cooling rates. On the other hand, if it takes over one hour to TABLE 3 Alloy element, percent Solution heat treatment temperature, 0.

Sam 10 num l aer C Cr Ni 1,050 1,100 1,150 1,200 1,250 1,300 1,350

301 0.41 20. 51 15.3 IV IV 111 III III Melt.-- Melt.

0.43 25.82 20.4 IV IV IV IV III "-110--- D0. 1103......- 0.24 25.13 35.4 IV IV IV IV III III Do. 304 0.12 27.56 39.5 IV IV IV 1V III III Do;

If the heating treatment is too low, part of the carbide will remain undissolved and, during the subsequent cooling, the undissolved carbide will serve as a nuclei for complete the 1,208 C. to 950 C. temperature transition, it will adversely affect the strength so that it may be concluded that cooling should occur over a period of carbide precipitation, resulting in a structure as shown from 5 seconds to one hour.

illustration only and are not intended to be limiting in any manner unless otherwise specified.

EXAMPLE 1 Table 8 shows the results of tests useing steel containing 25% Cr and Ni. The conventional forging material, i.e., Sample No. 81, has an extremely low strength than high carbon casting material, i.e., sample No. 80. Samples Nos. 82 and 84 which have been subjected to the normal TABLE 9 (EXAMPLE 2) Alloy element, percent Type 01 Ni working Heat treatment Creep rupture strength (kg/mm?) C Si Mn 01' 800 0. 900 C. 1,000 C. 1,050 C. Remarks 0.40 1.32 1.06 24. 91 19.92 Casting 6. 3.80 1.90 1.30 Comparative steel. 0.04 0.32 1.53 25.12 20.37 Forging-.- 1,100 C.,1hr.,W.Q, 3.75 2.05 0.80 0.70 Conventionalsteel' 0.43 0.37 1.24 25.82 20.41 ..do 1,280 C.,30min.,W Q 4.50 2.55 0.95 1.00 Comparativesteel. 0.43 0.37 1.24 25.82 1 .,30 6.60 3. 85 2.00 1. 26 Steel otthe present invention. 0.82 0.45 1.04 25.45 0. 90 Comparative steel. 0.82 0.45 1.04 25.45 1.80 Steel 01 the present invention. 0.31 0.52 0.97 25.46 Q 1.35 D0. 031 0. 52 0. 97 25.40 20. 59 .....do 1,280 o.;30'min.-(10 1.80 Do.

min.)-1,100 0., 15 mln., W.Q.

Creep rupture strength in 1,000 hours.

solution heat treatment show higher strength than the 25 XAMPLE 3 low carbon sample No. 81, but they are extremely inferior in strength to the casting material of Sample No. 80.

In contrast to this, samples Nos. 83 and 85 show extremely high strengths as compared to samples Nos. 81, 82

Table 10 shows test results of steels containing 15% Cr and Ni and Table 11 shows such results for steels containing 19% Cr and 41% Ni, respectively. The low carbon forging steels of Samples Nos. 1101 and 1201 and and presenting substantially the same strength as that 30 the forging steels of Samples Nos. 1102, 1104, 1201 and of the casting material sample No. 80.

1204 containing over 0.15% of carbon and subjected to TABLE 8 (EXAMPLE 1) Creep rupture Alloy element, percent T I strength (kg/mm?) V1 0 No. C Si Mn Cr Ni working Heat treatment 900 C. 1,000 0. Remarks 80 0.55 1. 24 1.13 25. 72 15. 10 Casting 2.40 1. Comparative steel. 81.-- 0.06 0.38 1. 58 25. E3 15. 58 Forging.-- 1,100 0., 1 hI'. W.Q.... 1,10 0, Conventional steel, 82 0.54 0.42 1.33 25.35 15.74 do 1,280 0., 3011 1 W.Q 1.70 1.10 Comparative steel. 83 0.54 0.42 1. 33 25.35 15.74 o-.. 1, 0., 30 nun. (30 min.)-1,050 0., W.Q. 2.40 1.60 Steel 01 the present invention. 84 0.94 0.61 1.01 24.93 15.01 ---d0-- 1,280 C-,30 111111., W. 1.80 1.15 Comparative steel. 85--- 0.94 0.61 1. 24.93 15.01 ...(10 1,280 0., 30 nun-(30 min.)-1,050 0., W.Q. 2. 50 1.65 Steel of the present invention:

Creep rupture strength in 1,000 hours.

EXAMPLE 2 conventional solution heat treatment, are extremely low Table 9 shows test results using steels containing 25% Cr and 20% Ni. Samples Nos. 92 and 94, which are high carbon steels, have been subjected to conventional solution heat treatment after forging. The conventional low carbon forging steel, sample No. 91, are all lower in strength than the high carbon cast steel of sample No. 90.

in strength as compared with high carbon casting steels of Samples Nos. 1100 and 1200. In contrast to this, steel Samples Nos. 1103, 1105, 1203 and 1205 of the present invention, afford the same degree of strength as that of the high carbon casting steels of Samples Nos. 1100 and 1200.

TABLE 10 (EXAMPLE 3) Creep Alloy element, percent rupture Type 01 strength Cr Ni working Heat treatment (kgJmmfi) Remarks 14.95 35. 18 Casting 2.20 Comparative steel. 15.18 36.03 Forging... 1,150 0., 1 hr., W.Q 0. 70 Conventional steel. 15.26 35. 72 -do 1,280 0., 1 hr., W.Q 0. Comparative steel. 15.26 35.72 ..do. 1,280 0., 1 hr.-(5 min.)-1,050 C., W.Q. 2.15 Steel 01' the present invention. 15.47 35.50 do. 1,280 C., 1 hr. W.Q 0.90 Comparative steel. 15.47 35.50 d0 1,280 0., 1 hr. (5 min.)-1,050 C., W.Q 2.10 Steel of the present invention.

1,000 hr. creep rupture strength at 1,000 0.

TABLE 11 (EXAMPLE 3) Alloy element, percent Type of strength No. C Si Mn Cr Ni working Heat treatment (kg/mm!) Remarks 1,2 .41 1.48 1.25 18.62 40.73 Casting 1.80 Comparative steel. 1, .05 0.28 0.86 19. 23 41.30 Forging 1,15 0., 1 hr ,W.Q- 0.65 Do. 1,202 .15 0.20 0. 65 18. 92 40. 68 1, C 1 hr ,W. 0.75 Do. 1,203 .15 0. 20 0.65 18.92 40.68 1, C.,1hr.- 15 1. 85 Steel otthepresentinvention. 1, .30 0.22 0.93 19.75 .73 1, C 1hr.,W.Q 0.80 Comparative steel. 1,205 0.30 0. 0. 93 19.75 41.73 1, C 1 hr min.)-1,050 C.,W.Q, 1. 80 Steel olthe present invention.

1,000 hr. creep rupture strength at 1,000 C.

In contrast to this, samples Nos. 93 and 95 present the same degree of strength as that of a high carbon steel No. 90. Samples Nos. 96 and 97 are shown as special EXAMPLE 4 This invention has been essentially described by reference to C-high Cr-high Ni type stainless steel in Examexamples of the present invention. Sample No. 96 was 75 pics 1 to 3. Example 4 illustrates the test results of a W, Mo, Nb, Ti, Al and N, separately. Table 13 shows the test results of the steels containing 27% Cr and 33% Ni, which steels further contain at least two elements of the 27% Cr and 33% Ni, which further contain each of Co,

elements enumerated above.

TABLE 14 (EXAMPLE 7) Creep Alloy element, percent T f ruptiire 1'0 0 strennt 1 C Si Mn Cr Ni Others working Heat treatment (hr.) Remarks 0.29 0.47 1.12 26.75 33.02 Forging... 1,2(810) 0., 161161 0 W Q 102. 3 Steel of the present min. 5 invention. 0.26 0. 28 1.16 25.04 33.17 00 15.45 Casting 160. 4 Comparative steel. 0. 06 0. 67 1. 51 25. 27 33. 38 C015.49 Forging..- 1,150 C., 1 hr., W.Q 25. 7 Do. 0.26 0. 28 1.16 25.04 33.17 Co 15.45 Extrusion- 1,280 C., 30 n1ln.,W.Q,...- 53.8 Do. 0.26 0.28 1.16 25.04 33.17 Co 15.45 ...do 1,2(Fi0 C., W Q 158.0 Steel oi the present 0min. 0 invention. 0.24 0. 39 1. 10 27.14 32. 75 Co 26.19 Casting 200. 4 Comparative steel. 0.24 0.39 1.10 27.14 32.75 Co 26.19 Extrusion. 1,280 C., 10 min.,W.Q... 65.2 Do. 0.24 0.39 1.10 27. 14 32. 75 Co 26.19 ...110 1,( C., min C W Q 241. 9 Steel of the present 10 min. ,050 invention. 0.31 0.61 1. 04 26. 53 33.10 W 5.32 Casting 190. 2 Comparative steel. 0.06 0.41 1.53 27.04 31.22 W 5.24 Forging... 1,150 C., 1 hr.,W.Q 35.9 Comparative steel. 0. 31 0. 61 1. 04 26. 53 33.10 W 5.32 Extrusion. 1,280 C., min.,W.Q- 43. 0 D0. 0.31 0.61 1.04 26.53 33.10 W 5.32 ...do 1,23% C., W Q 193.4 Steel of the present min. .0 invention. 0.24 0. 51 1. 63 25.85 31.82 Nb 3.23 Casting 221.5 Comparative steel. 0.07 0.85 0.82 26.15 32.41 Nb 3.57 Forging... 1,150 C., 1 ln'.,W.Q 32.1 Do. 0. 24 0.51 1. 63 25.85 31. 82 Nb 3.23 Extrusion. 1.280 C., 30 1nin.,W.Q...- 47. 3 Comparative steel. 0. 24 0. 51 1. 63 25. 85 31. 82 Nb 3.23 (lo 1,2(81% C., 2)301m5in .-C W Q 215. 2 Steel of the present min. ,0.0 invention. 0. 28 0.32 1. 26.21 32. 53 Mo 4.96 Casting 144. 8 Comparative steel. 0. 28 0. 32 1. 35 26. 21 32. 53 M0 4.96 Extrusion- 1,280 C., 30 Tnin.,W.Q,.-.- 35. 2 Do. 0. 28 0.32 1.35 26.21 32.53 Mo 4.96 ...do 1,2(80 C., .330 min ;c W Q 123.5 Steel of the present 10 min. -1,050 invention. 0. 31 0. 28 1.32 25. 85 33.28 Ti 2.62 Casting 138. 1 Comparative steel. 0. 31 0. 28 1. 32 25. 85 33. 28 Ti 2.62 Extrusion. 1,280 C., 30 min.,W.Q,..-- 35. 3 Do. 0. 31 0. 28 1. 32 25. 85 33. 28 Ti 2.62 ..do 1,28%" C., 30 161111 W Q 141. 2 Steel 01 the present min. -1, 1., invention. 0.30 0. 76 1. 43 26. 33. 44 Al 2.53 Casting 140.6 Comparative steel. 0.30 0.76 1.43 26.60 33.44 Al 2.53 Extrusion- 1,280 C., 30 min.,W.Q...- 35.3 Comparative steel. 0.30 0.76 1.43 26. 60 33. 44 A1253 ..do 1,280" C.,)301nin E3 W Q 158.0 Steel of the present 10 min. -1,050 invention. 0. 28 0.65 1. 44 27.07 33.93 N 0.32 Casting 138. 5 Comparative steel. 0. 28 0. 1. 44 27. 07 33. 93 N 0.32 Extrusion. 1,280 C., 1 hr., W.Q 36. 8 D0. 0. 28 0. 65 1. 44 27. 07 33. 93 N 0.32 ...do 1,280 0., 30 min.- 121. 7 Steel of the present (10 min.)-1,050 C.,W.Q. invention.

1,050 C., 25 kg. 'mm.

TABLE 15 (EXAMPLE 8) Cree Alloy element, percent T f truptulifg ype o s iengt No. C Si Mn Cr Ni Others working Heat treatment (hr. Remarks 1,500, 0.37 0. 34 0. 82 27. 09 32. 93 17.69 C0, 5-69 W Forging 613. 6 Comparative steel, 1,501 0.05 0.50 1.59 26.82 33. 64 16.36 Co, 6.02 W ...(10 1,150 C., 1 hr., W.Q 40.3 D0. 1,502 0. 37 0. 34 0. 82 27. 09 32. 93 17.69 Co, 5.69 W Extrusion- 1,280 C., 30 min., 7.111.... 52. 3 Do. 1,503. 0.37 0.34 0.82 27.09 32. 93 17.69 Co, 5.69 W -.-do 1,280; C., w Q 603. 7 Steel ofgahe present mm. inven ion. 1,510 0 32 0.56 1. 61 25. 96 31. 65 3.26 W, 1.56 Nb Casting 309. 6 Comparative steel.

, 11 32 0. 56 1. 61 25. 96 31. 65 3.26 W, 1.56 Nb Extrusion- 1,280 C., 30 min., W.Q.-. 46. 5 D0. ,512 0 32 0.56 1. 61 25. 96 31. 65 3.26 W, 1.56 N1) ...(10 1,2312; C W Q 323. 5 Steel oftthe present min. inven ion. 1,520 0. 29 0. 0a 1.18 20. 24 as. 27 2.20 w, 2.27 M0 Casting .-1 350.8 Comparative steel. 1,521. 0.63 1. 18 26.24 33. 27 2.29 W, 2.27 Mo Extrusion. 1,280 0., 30 min., W.Q..-. 40. 5 Do. 1,522 0 29 0.63 1.18 26.24 33.27 2.29 W, 2.27 Mo ...(10 1,2(310; C W Q 456.8 Steel oftthe present min. 1 1,530 0 31 0.56 1.23 27.77 35.90 2.45 Al, 1.97 Ti Casting 392.1 Coiiig afi'zitgge steel. 1,531 0 31 0.56 1.23 27. 77- 35.90 2.45 Al, 1.97 Ti Extrusion. 1,280 C., 30 min., W.Q,... 39.5 D0. 1,532 0 31 0.56 1.23 27. 77 35.90 2.45 Al, 1.97 Ti o 1,280;5(g ,0nv1vi1g(10n1in.) 398.5 Steel oftthe present inven 1o 1,540 0. 28 0.25 1.00 27.83 34.15 5.38 M0, 045 N Casting 285. 7 Comparati ire steel. 1,541 0.28 0.25 1.69 27.83 34.75 5.38 Mo, 0.45 N Extrusion. 1,280 C., 30 min., W.Q.... 45.3 D0. 1,542 0. 28 0.25 1.69 27.83 34. 5.38 Mo, 0.45 N -..do 1,2Ei085(03 ,0 13711600 min.) 370.3 Steel oft the present rnven 1on. 1,550 0.38 0.18 0.73 27.40 33.32 17.08 00, 7.27 W, 2.33 M0 Casting .3 704.4 Comparative steel, 1,551 0.38 0. 1.8 0.73 27.40 33.32 17.08 Co, 7.27 W 2.33 M0 Extrusion- 1,280 C., 30 min., W,Q,. 43. 8 Do. 1,552 0.38 0.18 0.73 27.40 33.32 17.08 C0, 227W, 2.33 M0 -.-d0 1,28055g C30 ret na-(15 min.) 689.9 Steel offthe present 1 mve 1,560 0.30 0.23 1.52 26.94 33. 04 15.35 Co, 5.35 W, 0.94 Nb Casting 765. 4 Compa t rzigi ile steel, 1,561 0.30 0.23 1.52 26.94 33.04 15.35 Co, 5.35 W,0.94 Nb Extrusion. 1,280 C., 30 min., W.Q...- 47.3 D0. 1,562 0.30 0.23 1.52 26.94 33.04 15.35 00,5.35 W, 0.94 Nb -....do...-. 12803565 301 1 1 05 min.) 883.5 Steel oftthe present mve 1,570 0.27 0.27 1. 38 26.79 32.63 5.22 W, 2.32 Mo, 1.02 Nb Casting 388.1 compgnigi ire ste l, 1,571 0. 27 0.27 1.38 26.79 32. 63 5.22 W, 2.32 Mo, 1.02 Nb Extrusion- 1,280 0., 30 min., W.Q.-.. 42. 0 D0. 1,572 0.27 0.27 1.38 26.79 32.63 5.22 W, 2.32Mo, 1.02 Nb ..-do 1281085 (130215 1305 min.) 392.6 Steel of the present inve tio 1,580 0. s7 0. as 1. 40 27.68 32.81 15. 4 .5 5 W, 2.70 Casting 867.2 compgt rati im Steel, 1,581 0.37 0.35 1.40 27.68 32.81 l5isI3 gg,5 W, 2.79 Extursion. 1,250 0., 1l1r., W.Q 50.6 Do.

0 1,582 0.37 0.35 1.40 27.68 32.81 15%(3155 W, 2.79 ...do 1,250 C., 1 Ina-(15 min.) 910.3 Steel oft the present mve 1,590 0. a2 0. 54. 1. e0 20. s0 30. 75 1618 19) 3 5,10 W, 2.32 Casting 656.8 comp ragi ire St 1,591 0.32 0.54 1.69 26. 33.75 111 9'g s s0 W, 2.32 Extrusion. 1,250 C., 1 hr., W.Q 45.6 Do.

, .1 o, 1,592 0.32 0.54 1.69 26.80 33.75 16.89 00 5.39 S, 2.32 do 1,250 C., 1 hr.-(15 min.) 594.3 Steel of the t Mo, 0 12 N 4,050 0., W.Q. invention. presen The test results are illustrative of the superb characteristics of high creep rupture strength obtainable with the alloy of the present invention. Accordingly, it may readily be appreciated that the heat resistant steels of the present invention will find application in producing reformer tubes and cracking tubes, as well as forming heat exchanger tubes, wherein high strength is required at temperatures as high as 850 C., or higher, which cannot be achieved by the conventional 0.01 C-25Cr20 Ni stainless steel or Incolloy 800 alloys. These achievemerits can be attributed both to the elevated temperature strength of steels of the present invention, and of their capabilities of providing thinner wall thicknesses as required in such reformer and cracking tubes and in heat exchanger tubes.

In addition, it is to be understood that the steels of the present invention exhibit excellent characteristics in the application to forgings, rollings and extrusions. Thesecharacteristics permit the production of tubes having outer diameters of mm. and yet wall thickness of even below 1 mm., which is particularly suited for use as heat exchanger tubes, and they further permit the production of tubes having a length of over 10 m., thus minimizing the number of welding joints which tend to cause trouble during the use of the tubes.

It can still further be recognized that the steel of the present invention provides good, sound layers on the inner surface of the tube, thus permitting the use of a tube having a thinner wall thickness with resulting good heat capability, while eliminating the likelihood of rapid carburization of the reducing gas due to imperfections in the tube surfaces.

In addition, the methods of the present invention permit the production of sheet form steel by ordinary rolling processes.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

What is claimed and intended to be covered by Letters Patent is:

1. An austenite type heat resistant steel article characterized by high strength properties at elevated temperature consisting essentially of from 0.1 to 1% by weight carbon, from 0.01 to 3% by weight silicon, from 0.01 to 10% by weight manganese, from 13 to 35% by weight chromium, from 15 to 50% by weight nickel, and the balance essentially impurities and iron, wherein a continuous carbide precipitation formed by heat treatment at a temperature of greater than 950 C. is present on the grain boundaries.

2. The austenite type heat resistant steel of Claim 1, wherein said steel additionally contains at least one element selected from a group consisting of up to 30% cobalt, up to 10% tungsten, up to 10% molybdenum, up to 5% niobium, up to 5% titanium, up to 5% aluminum and up to 0 .5% nitrogen.

3. A process for producing an austenite type heat resistant steel characterized by high strength at elevated temperatures, said steel containing, in weight percent, from 0.1 to 1% carbon, from 0.01 to 3% silicon, from 0.01 to 10% manganese, from 13 to 35% chromium, from 15 to nickel, and the balance essentially impurities and iron, which comprises working said steel, heating said worked steel to a temperature of from 1,150 C. to the solidus line, slow cooling said steel from said temperature down to a temperature of 950 C. or higher, over a period of from 5 seconds to one hour and thereafter rapidly cooling said steel.

4. The process for producing an austenite type heat resistant steel as defined in Claim 3, wherein said steel additionally contains at least one element selected from a group consisting of up to 30% cobalt, up to 10% tungsten, up to 10% molybdenum, up to 5% niobium, up to 5% titanium, up to 5% aluminum, and up to 0.5% nitrogen.

5. A process for producing an austenite type heat resistant steel characterized by high strength at elevated temperatures, said steel containing, in weight percent, from 0.1 to 1% carbon, from 0.01 to 3% silicon, from 0.01 to 10% manganese, from 13 to 35% chromium, from 15 to 50% nickel, and the balance essentially impurities and iron, which comprises casting said steel, heating said cast steel to a temperature of from 1,150 C. to the solidus line, slow cooling said steel from said temperature down to a temperature: of 950 C. or higher, over a period of from 5 seconds to one hour and thereafter rapidly cooling said steel.

References Cited UNITED STATES PATENTS 3,329,535 7/1967 Langer et al. 148-123 3,437,477 4/ 1969 McCune H1 -128 R 2,879,194 3/ 1959 Eischelberger 148--142 3,199,978 8/1965 Brown et a1 14838 3,243,287 3/1966 'Lillys et al. 75-171 3,420,660 1/1969 Kawahata et al. 75-128 R 3,459,539 8/1969 Eiselstein et al. 75-128 R WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R.

Page of 3 UNITED STATES PATENT AND TR l OFFHJE QRTTFTCT EEQTTN PATENT NO. 3,826,689 DATEB July 30 l974 INVVENTOMS) SADAO OHTA ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 line l "0.50C-25Cr20Ni should read --O.50C-25Cr-20Ni Column 3, line 6, "manganes" should read --manganese--.

Column 4, line l0, "as" should read --at--;

line 62, "provent" should read --prevent--.

Column 6, Table 4, third from the last column 2.85 min. should read --28.5 min,

Column 8, fourth line after Table 7, delete "hi bit" and insert hi bits--..

fifth line after Table 7 delete "show" and insert --shows--.

Column 9, line 6, delete low" and insert --lower--;

Table l0 (Example 3) in the Column labeled "No. correct the alignment of the numbers.

line 73, after "carbon" insert --cast--.

Columns ll and l2, Table l5 (Example 8) in the row of No. l 58l delete "Extursion" and insert --Extrusion--;

- in the row of No. l ,582

after "l,250C., lHr.-(l5 min.) insert -lO50C., N.Q,--.

The sheets of drawings appearing in the patent should be cancelled and the sheets shown on the attached sheets should be substituted therefor, but will apply to the Grant onlyw floated this fourth D3) f May1976 igned [sum - Arrest:

RUTH C. MASON (I. MARSHALL DAMN Atlesting Officer ('nmmissr'mwr nj'lamrrs and Trademarks Patent No. 5,826,689 Page 3 of 3 FIG.2(I) F1 2 FIG.2(11I) 

